1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus, and particularly to a process of using a patch to determine a conversion relationship for a color transformation process relating to data generation for color materials such as ink used in an image output apparatus such as an ink jet printer.
2. Description of the Related Art
In general, when an image output apparatus such as a printer is used to print and output a color image, four color materials for Y (Yellow), M (Magenta), C (Cyan), and K (black) are used. For example, an ink jet printer uses Y, M, C, and K inks. Further, a copy machine or a printer of an electrophotography uses Y, M, C, and K toners.
A color space realized by these color materials, used in the image output apparatus, is normally different from a color space for an original image data. Thus, in the field of image processing in which color material data is generated on based on image data, various image processing methods have been proposed to reproduce faithfully colors or gradations shown by the original image data or to reproduce desired colors and the like.
Among these image processing methods, for example, Japanese Patent Application Laid-Open No. 2-136848 (1990) describes an image processing method that can eliminate the color reproducibility problem associated with a 100% UCR (Under Color Removal) process executed as a part of the color material data generating process. In this method, of various combinations of Y, M, C, and K which can express a color shown by the original image data, those in which K has the highest density are defined to be the combination of Y, M, C, and K. According to this method, in contrast to the 100% UCR, in which one of Y, M, and C which has the lowest density is replaced with K to have a value of 0, combinations of Y, M, C, and K which do not include a value of 0 maybe present. Consequently, an image with no decrease in a saturation or density can be outputted. Furthermore, compared to the image processing described in the above publication, Japanese Patent Application Laid-Open No. 6-242523 (1994) describes an image processing method of setting a value for K taking into account the reproduction ranges of the color materials Y, M, C, and K, a pseudo contour of an output image, interpolation accuracy, and the like to further improve reproducibility.
The above described image processing relating to the color transformation is commonly carried out using a lookup table (LUT) having data used to carry out the color transformation as well as interpolation. The contents of the table are determined by printing patches for a plurality of combinations of the color materials Y, M, C, and K and subjecting the patches to a colorimetric operation. Specifically, if signal values for the respective color materials Y, M, C, and K constituting each patch data are expressed by 8 bits, then for example, patches are printed for 4,096 colors obtained by combining signal values of respective four color materials, each of which is one of 0, 32, 64, . . . , 223, and 255 that are obtained by equally dividing 256 values for the respective colors into eight parts. Then, the patches are subjected to a colorimetric operation, and on the basis of the results of the colorimetric operation, a combination of Y, M, C, and K that reproduces a predetermined target is determined to be table data.
However, even if image processing is executed so as to improve the reproducibility of the image output apparatus such as a printer as described above, actual printing in the printer or the like becomes improper owing to the relationship between the color materials and printing sheets. As a result, images cannot be reproduced as desired.
The processes described in the above two publications simultaneously use the four color materials Y, M, C, and K and thus tend to use a large total amount of color materials. In particular, if a color with a high saturation is expressed at a low lightness, each color material has a high signal value. Consequently, a large amount of color material must be used. In this case, in an ink jet printer, a printing sheet may not sufficiently absorb ink depending on the ink absorption characteristic of the printing sheet. Then, the ink may overflow or bleed, thus hindering density or the like from being accurately realized. Further, with an electrophotographic printer using toner, the toner may not be sufficiently fixed to a printing sheet depending on the toner adhesion characteristic of the printing sheet. Again, the density or the like cannot be accurately realized. As a result, even if image reproducibility is intended to be very high in the image processing for generating color material data, images may not be accurately reproduced in actual printing.
When patch data, described above, is generated and printed on the basis of image processing such as that described above, then similar problems may occur. In particular, in patch printing, patch of data inhibited in normal printing may be printed. Thus, the problem becomes more marked. For example, if the maximum amount of each color material is assumed to be 100% and signal values are such that all the color materials use the maximum amount, then the total amount of color materials applied to the printing sheet is 400%. If the four color materials are used so that their amounts are close to the maximum value, then in the ink jet printer, the printing sheet cannot absorb four types of inks applied so as to be superimposed on one another. Further, the electrophotographic printer using toners cannot appropriately fix the four color toners to the printing sheet. As a result, patches can not be reproduced accurately.
If the patches are thus not accurately printed, colorimetric values, as a matter of course, do not reflect the printing characteristics of a printer that has printed the patches. This makes it impossible to generate the table for accurate color transformation.
Of course, the problem with the amount of color materials used depends on the relative characteristics between printing sheets and the color materials, such as absorption and adhesion characteristics. Accordingly, if the amount of color materials used is, for example, 300%, a similar problem may occur depending on the printing sheets used. Further, even if the total amount of color materials used is based on the above described 100% UCR, a similar problem may occur depending on the relative relationship between the print sheets and the color materials.